1. Field of the Invention
This invention relates generally to apparatus for recording and/or reproducing video signals, and more particularly is directed to improvements in such apparatus of the type known as a helical scan VTR or video tape recorder.
2. Desription of the Prior Art
Various types of video tape recorders, hereinafter called VTRs, have been developed, for example, the so-called 4-head Ampex-type or the helical scan-type which may employ a .OMEGA.-wrap of 180.degree. or 360.degree. or an .alpha.-wrap. The most popular of the helical scan VTRs is the 180.degree. .OMEGA.-wrap with two rotary recording or reproducing magnetic heads. Generally speaking. helical scan VRTs have many advantages over the 4-head Ampex type VTRs. More particularly, helical scan VTRs can be easily changed-over between various playback or reproducing modes, such as, a slow-motion mode, a quick-motion or speeded-up mode and a stop or still-motion mode.
Among the existing helical scan VTRs, the 180.degree. .OMEGA.-wrap type is advantageous, as compared with the 360.degree. .OMEGA.-wrap or .alpha.-wrap type in that a relatively small angular extent of the tape guide drum surface is engaged and, therefore, the frictional resistance to longitudinal movement of the magnetic tape relative to the guide drum is correspondingly reduced. Such reduced frictional resistance to longitudinal movement of the tape correspondingly reduces the stretching of the tape during playback or reproducing and skew distortion in the reproduced video picture can be minimized. Further, it is difficult to maintain smoothness of the tape on the guide drum surface in the case of a 360.degree. .OMEGA.-wrap or .alpha.-wrap.
Although 180.degree. .OMEGA.-wrap type VTRs have had extensive acceptance for home and industrial or educational uses, problems are encountered in adapting this type of VTR for use in the broadcasting field which requires a relatively large frequency range. It will be noted that the frequency range of a VTR is dependent on the speed of each rotary head relative to the tape during recording and reproducing or playback operations. In the usual 180.degree. .OMEGA.-wrap type VTR having two substantially diametrically opposed rotary heads and intended for use with NTSC video signals, the heads are rotated at a speed of 30 revolutions per second so that the two heads will alternately record or reproduce successive fields of video signal information in respective parallel tracks extending obliquely across substantially the entire width of the magnetic tape. In other words, each track extending obliquely across substantially the entire width of the tape has a field interval recorded therein. If the rotational speed of the rotary heads is retained at 30 revolutions per second, then an increase in the relative head-to-tape speed, for example, for expanding the possible frequency range to the extent necessary for broadcast use, can be achieved by very substantially increasing the diameter of the tape guide drum and of the circular path followed by the rotary heads in recording and reproducing operations. However, increasing the diameter of the tape guide drum correspondingly increases the overall dimensions of the VTR and makes it difficult to design the latter as a portable machine.
In order to increase the relative head-to-tape speed in a 180.degree. .OMEGA.-wrap helical scan VTR without increasing the tape guide drum diameter, it has been proposed to divide each field of the recorded video signal into a plurality of segments which are recorded in respective successive parallel tracks each extending obliquely across substantially the entire width of the tape. Thus, for example, the rotational speed of the rotary head or heads may be increased so that each of the oblique tracks extending across substantially the entire width of the magnetic tape will contain recorded signal information corresponding to only one-third of a field interval, that is, three successive tracks will be employed for recording one respective field of the video signals. Although the foregoing permits an increase in the head-to-tape speed for expanding the frequency range without undesirably increasing the tape guide drum diameter, the use of two or more successive oblique tracks for recording each field of the video signals requires head switching at one or more points in each the field intervals during recording and reproducing operations. In the case where the plurality of successive tracks in which each field of the video signals is recorded extend obliquely across substantially the entire width of the tape, there is a relatively large distance along the tape between the head switching points, that is, between the point on one track where reproducing therefrom is terminated and the point on the next track where reproducing therefrom is commenced. Thus, when the magnetic tape stretches after recording, a significant change can occur in the distance between the head switching points and the resulting so-called jumping jitter due to skew distortion may exceed the horizontal blanking interval when reproducing the video signals so as to cause discontinuity of the resulting reproduced image or picture on the screen of a monitor or television receiver and deterioration of the color fidelity when reproducing color video signals.
By reason of the foregoing, helical scan VTRs employed for broadcasting use have usually been of the 360.degree. .OMEGA.-wrap or .alpha.-wrap type, for example, as disclosed specifically in U.S. Pat. No. 3,188,385, having a common assignee herewith. In such apparatus, a single rotary head is employed for recording and reproducing successive fields of video signals in respective successive tracks extending obliquely across substantially the entire width of the magnetic tape. Since a complete field interval is recorded in each of the tracks, the rotary speed of head is 60 revolutions per second for desirably increasing the head-to-tape relative speed and, furthermore, the head switching points occur in the vertical blanking intervals which are not exceeded by the possible jumping jitter. However, as previously noted, helical scan VTRs of the 360.degree. .OMEGA.-wrap or .alpha.-wrap type are generally undesirable in that there is relatively large frictional resistance to longitudinal movement of the tape and it is difficult to maintain smooth engagement of the tape with the guide drum.